What is a Restriction Orifice?
Restriction orifice or RO, in short, is a flow control instrument device whose primary function to provide a restriction to the flow so that a controlled or restricted flow is achieved. Due to this restriction by the orifice plate, a pressure head drop from the upstream of the orifice to the downstream is observed. For a specific temperature and pressure condition, the orifice area at the outlet determines the volumetric flow rate of the fluid inside the pipe. Due to the high pressure drop at the restriction orifices, it can create acoustic induced vibration. Hence, studies must be performed by specialists.
Orifice plate is basically a thin plate with an orifice in the middle. The plate is inserted between two flanges of a pipe for providing restriction or flow measurement.
Purpose of Restriction Orifice
Restriction Orifice is used primarily for two main reasons:
- If there is a requirement of Reduced flow or
- If there is a requirement of high pressure-drops.
The device is sized depending on the required pressure drop. They are designed to slip between the piping flanges. Restriction orifice comes with single restriction orifices or multiple restriction orifices in Series.
Applications of Restriction Orifice
Engineers and designers are familiar with a restriction orifice in the following applications for impeding flow or reduce pressure.
- They are installed downstream of blowdown valves to ensure a controlled flow rate in the blowdown piping or blowdown header.
- They are installed in the minimum flow bypass lines around centrifugal pumps.
- They are installed in Wellhead applications
- Restriction Orifice is sometimes used to restrict excess flow in case ruptures.
- Minimum bypassing
- Injection, cooling, and flushing of fluid.
- The steam let down.
- Use as a simple static mixer
- N2 purge or constant gas seal
- Controllability improvement
Restriction Orifice Standard
There is no direct standard addressing the design of restriction orifice, but some associated references are available as listed below:
- ISO 5167 Part 1 and Part 2
- ISA RP 3.2
- API -RP 550/551
- API 2531
- IEC 60534-8-3
- API Manual of Petroleum Measurement – Chapter 4
- AGA Report No.3
- API MPMS 14.3.2
- ISO 5024
- ISO 5168
Working Principle of Restriction Orifice/Orifice Plate
Both Restriction Orifice and Orifice plate work following the Bernoulli’s principle that states that pressure drop across the restriction orifice is directly proportional to the volumetric flow rate passing through the orifice plate.
While fluid flows through the plate, fluid velocity changes that in turn, as per Bernoulli’s equation change the pressure. This change in pressures upstream and downstream is measured to calculate the volumetric flow rate.
Restriction Orifice Types
Restriction Orifice Plates are of three types
- Single Stage Restriction Orifice
- Multi-hole Single Stage Restriction Orifice
- Multistage Restriction Orifice Assembly
Inputs for Restriction Orifice Design
The following inputs are required for the design of restriction orifice:
- PFD and P&ID
- Hydraulic Calculation Sheet
- Line operating conditions like upstream-downstream pressure, temperature, flow rate, line size, Density, Viscosity, molecular weight, Cp/Cv, vapor pressure, etc.
Restriction Orifice Design Steps
The design of Restriction orifice or RO is carried out as per the steps similar to as mentioned below:
- Determination of Application
- Data Preparation for the orifice
- Restriction orifice sizing (by design engineer or vendor)
- Checking of critical design elements like cavitation index (Kc=0.93), pressure drop, minimum orifice diameter, allowable space in piping routing, etc.
- Filling out the necessary information in P&ID and RO datasheets
Installation of Restriction Orifice
The restriction orifice is physically a thin plate with one or more holes in it. In piping application, They are normally inserted in between two flanges. It is possible to inadvertently leave out the plate when the piping is assembled, or, more likely, to forget to replace it when the piping is reassembled, following maintenance or cleaning. So leaving out the orifice is difficult.
To prevent this, one should make it impossible to assemble or reassemble the piping without including the orifice. A simple, practical and foolproof method is: the restriction orifice has its own spool piece. So in such a situation, there will not be scope for forgetting the RO element during installation or Construction.
Orifice upstream and downstream requirements
It is a standard engineering practice to keep 10 pipe diameters (10D) upstream (before the orifice) and 5 pipe diameters (5D) downstream of the orifice.
Restriction Orifice Symbol
The following symbols are used in P&ID to describe restriction orifice and orifice plates.
Restriction Orifice vs Orifice Plate
The major differences between Restriction orifice and orifice are tabulated below:
|Purpose||Used for killing the high pressure or reducing pressure; a pressure reducing device.||Used for flow measurement.|
|Hole Profile||The hole of the restriction orifice has a straight profile||The hole profile of the orifice is straight at first but then bevelled (notched) with a 45° slope|
|Pressure Drop||High Pressure drop||Pressure drop is low.|
|Fluid Velocity||Sonic Velocity to ensure choked flow.||Subsonic flow.|
|Acoustic Induced Vibration||Highly Susceptible to AIV and high noise can generate||Not Susceptible to AIV.|
Few more Resources for you..
Reference: CHEMICAL ENGINEERING/ APRIL 13, 1987
About the Author: Part of this article is written by is Mr. Amir Razmi, an International, dynamic, and multi-functional chemical engineer with 14+ years’ experience in engineering and EPC of oil and energy projects from pre-contract activities to execution, and closeout.